Many monitor systems use an analog current transmitting sensor circuit in combination with a monitor or control circuit. For example, FIG. 1 illustrates a prior art monitor system 10 wherein exhaust flow 12 is monitored by a sensor, in this case pressure sensing current transmitter 14. Transmitted current 16 is proportional to the amount of exhaust 12, and is provided to exhaust display and monitor circuit 18. Monitor circuit 18 is a sensor controlled device which monitors and displays the exhaust level on exhaust display 19, and provides control of alarm 23 and other control circuits via a configurable exhaust level setpoint. A user may configure the exhaust level setpoint to trigger alarm 23 if the exhaust level drops below a certain point.
For system 10 of FIG. 1 to work correctly, monitor circuit 18 needs to have three configurable functions working properly: gain adjustment 25, zero adjustment 27, and setpoint adjustment 29. Gain adjustment 25 and zero adjustment 27 justify the exhaust display with the actual signal range provided by the monitored exhaust. Setpoint adjustment 29 determines the minimum exhaust level that will trigger alarm 23 and shut off controls.
To test prior art system 10 requires physically disconnecting sensor 14. There is then no transmitted current 16 flowing to monitor circuit 18. As far as monitor circuit 18 is concerned, this is equivalent to no exhaust flow 12. This triggers alarm 23, and provides the check that monitor system 10 is working correctly. Alternately, monitor system 10 may be tested by actually turning off exhaust flow 12.
The problem with this method of testing is that it only provides an on/off test. It does not test the calibration of monitor circuit 18, and does not indicate the exhaust level setpoint for alarm 23. In addition, both types of tests, physically removing sensor 14, or turning off exhaust flow 12, are often impractical and time-consuming.
Consequently, it would be desirable to have a method and apparatus for testing a sensor controlled monitor device that tests the calibration, as well as the exact value of any setpoints, of the monitor device. It would also be desirable to have a method and apparatus that are easy to use, and do not require physically disconnecting the transmitting sensor circuit or physically changing the amount of the substance being monitored.